Detection of crevice corrosion of steels in seawater by optical interferometry

In the present investigation, an optical corrosion-meter has been developed for materials testing and evaluation of different corrosion phenomena. The idea of the optical corrosion-meter was established based on principles of 3D-holographic interferometry for measuring microsurface dissolution, i.e. mass loss, and on those of electrochemistry for measuring the bulk electro nic current, i.e. corrosion current of metallic samples in aqueous solution s. In the present work, an early stage of crevice corrosion of a carbon ste el and a stainless steel in seawater was monitored in situ by the optical c orrosion-meter during the cyclic polarization test. The observations of cre vice corrosion were basically interferometric perturbations detected only o n the surface of the carbon steel and the stainless steel underneath a crev ice assembly, made of a Teflon bolt, Teflon nut, and a Teflon washer. The c revice assembly was used on all tested samples to create a differential aer ation cell between the surface of the sample and the areas underneath the c revice assembly in seawater. Each Teflon washer contained radial grooves an d had 20 plateaus that formed crevices (shield areas) when pressed against the surface of the sample. The interferometic perturbations interpreted as localized corrosion in the form of an early crevice corrosion as a depth ra nging between 0.3 mum and several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very useful tec hnique for monitoring crevice corrosion at the initiation stage of the phen omenon for different metallic samples in aqueous solutions. (C) 2000 Elsevi er Science Inc. All rights reserved.